The melanosome is the unique organelle of the vertebrate pigment cell and is the site of synthesis of melanin pigment. The genetics of vertebrate pigmentation have been studied extensively, especially in the mouse where a set of inbred pigmentation mutants with common genetic background is available. Some of the Mendelian genes which control pigmentation in the mouse appear to have their primary site of action within the melanosome. The goal of this project is to develop a "biochemical anatomy" of the melanosome especially in terms of its protein subunits and then to study the synthesis and assembly of these proteins to form melanosomes and finally to study the control of melanosome formation by some of the genetic loci which determine melanosome structure. Current progress in these studies is related to the use of two serially transplanted mouse melanotic tumors: black tumor B-16, genotype BB and brown tumor S-91, genotype bb which are genetically different from one another at the B locus discussed above. The biochemical genetics of the expression of pigmentation phenotype in the mouse has extremely important application in the study of embryonic development and differentiation as well as implications for the medically related study of malignant melanoma.